Electrically conductive adhesive materials are widely used in various biomedical applications. These materials are employed on biomedical electrodes, ground plates, and the like to establish an electrical connection between the skin of the human anatomy and an electromedical apparatus. Specifically, conductive adhesive material are used on electrodes for electromyographs, electrocardiographs, TENS (transcutaneous electrical nerve stimulation) systems, on ground plates for electrosurgery, and the like.
U.S. Pat. No. 4,066,078 to Berg describes conductive adhesives comprising an organic polymer plasticized with a polyhydric alcohol, wherein the polymer is derived from an ester of an olefinically unsaturated carboxylic acid and an alcohol having a quaternary ammonium group, or is derived from a sulfated cellulose ester.
Larimore et al., in U.S. Pat. No. 4,273,135, describe disposable biomedical electrodes having a conductive material which has no salt added for conductivity. The Larimore et al. material is a film forming, hydrophilic polymer selected from: (a) non-ionic water soluble polymers of substantially all water-soluble monomers; (b) non-ionic water soluble interpolymers of water-soluble monomers and water-insoluble monomers; or, (c) non-ionic hydrophilic water-insoluble interpolymers of water-soluble monomers and water insoluble monomers containing at least 15 mole percent of interpolymerizable water-soluble monomers. Preferred polymers from class (a) include polyvinylalcohol, polyvinylpyrrolidone, polyacrylic acid and the like. Preferred interpolymers from class (b) include copoly(vinylacetate:vinylalcohol) and copoly(n-butyl acrylate:acrylic acid). Preferred interpolymers from class (c) include copoly(vinylchloride:vinylalcohol). Larimore et al. suggest that the addition of tackifiers to their formulations is necessary in most cases to obtain good adhesive qualities. For example, the Larimore et al. formulations based on polyvinyl-alcohol and glycerin (with no additional tackifiers) were found to be non-tacky.
In U.S. Pat. No. 4,391,278, Cahalan et al. describe a conductive adhesive comprised of a polymerized form of 2-acrylamido-2-methyl-propansulfonic acid with water or an alcohol, e.g. glycerol. Cahalan et al. disclose that optional thickeners, such as polyvinylpyrrolidone, polyvinylalcohol, karaya gum, and xanthan gum, may be employed with their acid polymer composition.
U.S. Pat. No. 4,094,822 to Kater describes a biomedical electrode which makes electrical and physical contact to the skin by dispersing a metal salt, e.g. silver chloride, throughout an adhesive which is preferably 15 to 25 percent polyvinylalcohol (88% hydrolyzed form), 5 to 7.5 percent boric acid, 1.5 to 2.5 percent carboxymethylcellulose, 5 to 10 percent glycerol and the balance water. The conductivity of these and most other adhesive formulations is dependent on a relatively stable water content. Since the adhesive of Kater typically contains 55 to 70 percent by weight of water, special packaging is required to maintain that water content. Formulations having this much water are likely to partially dry out in ambient conditions, thereby adversely affecting the electrical and elastomeric properties.
Engel, in U.S. Pat. No. 4,554,924, describes a conductive adhesive prepared by forming an adhesive precursor on the electrode and thereafter polymerizing the precursor in situ. The conductive adhesive precursor comprises a water soluble polyhydric alcohol which is liquid at about 20.degree. C.; at least one non-ionic unsaturated free radically polymerizable material soluble in said polyhydric alcohol; a free radical initiator soluble in said polyhydric alcohol; a cross-linking agent of a multifunctional unsaturated free radically polymerizable material soluble in said polyhydric alcohol; and, an ionizable salt in an amount effective to render said adhesive product electrically conductive. The polyhydric alcohol can be glycerol and the polymerizable material is preferably acrylic acid, methacrylic acid, N-vinyl-pyrrolidone and the like. The initiator can be, for example, benzoyl peroxide; the cross-linking agent is preferably TEGBM (triethylene-glycol-bis-methacrylate), and the ionizable salt is typically an inorganic halide, preferably an inorganic chloride. These adhesives also may contain about 10 percent by weight of water.